In photolithography, overlay is defined as the layer to layer registration performance; overlay error is the runout error between layers at various sites on the wafer. In the fabrication of microminiaturized components, as for example in the production of solid state devices, thousands of the devices are produced in a single wafer of semiconductive material using a multi-step photomechanical reproduction process, by photolithography. The pattern depicted on the mask, by photo imaging, is applied to a repetitive array of a single element of a multi-element array required for the fabrication of the device, and then by a succession of alignment and fabricating steps, the completion of a finished product. Conventionally, Perkin-Elmer mask aligners are used in the achievement of these ends. Such Perkin-Elmer mask aligners are found to possess systematic overlay errors or overlay signatures. These overlay signatures are quite different from one aligner to the other, but depend upon the unique characteristic optical distortions of the particular tool.
Analyzing the overlay signatures from different Perkin-Elmer mask aligners, it was determined that each individual tool's signature can be further separated into its linear components, measured by its Y-axis magnification along the imaging annular ring field, and its non-linear component, measured by top to bottom magnification balance, local distortions and straightness of Y-axis.
Typically, there exist four kinds of signature error: i.e. magnification error of other than one to one; non-uniform magnification error over the field of exposure typically top to bottom, i.e. one-half field; non-uniform magnification over the field of exposure within a half thereof, i.e., one-fourth field of exposure; and, non-linearity of image, i.e. bow in the direction of the optical axis.
The linear component can be changed to match that of the base-line tool by tool-enclosure temperature tuning. However, the non-linear component of tool signature can be matched heretofore only with tools that have similar signatures. Vendor specification such as that of the (Perkin-Elmer corporation) only guarantees plus or minus 10.mu."/site to contact standard or .+-.20.mu."/site tool to tool. However, Applicant's assignee overlay specification, for 78 G. R. (Ground Rule) and 80 G.R., are .+-.10.mu."/site and .+-.7.mu."/site, tool to tool respectively. Currently, only 40 percent of existing tools are capable of meeting the 78 G. R. (or 15 percent for 80 G. R.).
It is, therefore, a primary object of the present invention to provide a method and apparatus for compensating distortions arising in an optical mask alignment apparatus to permit fine tuning of individual Perkin-Elmer or like overlay signatures to match those of base line tools for upgrading ground rule performance and to recover tools having mismatched signatures.